Merge pull request #1319 from asthetik/fix-warning

Eliminate compiler warnings

src/advance/functional-programing/iterator.md

@@ -435,7 +435,7 @@ let val = v.iter()
     // 每两个元素剔除一个
     // [1, 3, 5]
     .filter(|&(idx, _)| idx % 2 == 0)
-    .map(|(idx, val)| val)
+    .map(|(_, val)| val)
     // 累加 1+3+5 = 9
     .fold(0u64, |sum, acm| sum + acm);